1. Field of the Invention
The present invention relates to a step-up/step-down circuit with a simplified circuit configuration.
2. Description of the Related Art
Photoelectric conversion devices including photoelectric conversion elements have come to be widely used in recent years. Because devices are often incorporated into portable terminal devices, such as mobile phones, reduction in size of not just the photoelectric conversion devices, but also of peripheral power supply circuits, is desired.
For vertical and horizontal transfer and output of information electric charges produced in accordance with incident light, photoelectric conversion devices having a CCD solid-state image sensing element require a predetermined transfer clock pulse to be applied to vertical and horizontal transfer registers, a gate electrode, and a transfer electrode in an output unit. A crest value of such a pulse, i.e. a pulse voltage, is set at a predetermined voltage level by stepping up or down the voltage of a system clock of the photoelectric conversion device.
The photoelectric conversion device having the CCD solid-state image sensing element generally uses a system clock pulse of 0-3 V. The transfer clock pulse is produced by boosting or bucking the voltage of the system clock pulse. A step-down circuit includes a charge pump circuit as shown in FIG. 5. A main circuit unit 10 is formed as a semiconductor element. Often, capacitors Ca-Cd included in the step-down circuit are connected to external terminals Ta-Td outside the main circuit unit 10. In such a step-down circuit, a reference voltage terminal is connected to ground, and the system clock pulse of 0-3 V is applied to an input terminal, thereby obtaining potentials Va-Vd at connection nodes A-D, respectively, as shown in FIG. 6. More specifically, the voltage is reduced from the reference ground potential (GND) through the two-stage charge pump action, and the potential Vc of approximately −6 V is applied to a regulator 12, so that the regulator 12 can produce the output voltage Vd of approximately −4 V. On the other hand, a step-up circuit also includes a charge pump circuit as shown in FIG. 7 similarly to the step-down circuit. A main circuit unit 14 is formed as a semiconductor element. Capacitors Ce-Ch included in the step-up circuit are often connected to external terminals Te-Th outside the main circuit unit 14. In this step-up circuit, a system voltage VDD of 3 V is supplied to a reference voltage terminal, and the system clock pulse of 0-3 V is applied to an input terminal, thereby obtaining potentials Ve-Vh at connection nodes E-H, respectively, as shown in FIG. 8. More specifically, the voltage is boosted from the reference system voltage VDD through the two-stage charge pump action, and the potential Vg of approximately 9 V is supplied to a regulator 16, which converts the potential Vg into the output voltage Vh of approximately 7 V, and outputs the converted voltage.
Conventional step-down and step-up circuits such as those described above are generally formed as completely independent circuits for producing the desired output voltages Vd and Vh from the system clock pulse, a large number of components are required for the step-up/step-down circuits, thereby increasing the scale and size of the circuit. In addition, because a large number of external terminal pins for connecting to the capacitors Ca-Ch are required, the area occupied by pad electrodes for extracting the external terminals in the step-up/step-down circuit is increased, preventing further reduction in size of the photoelectric conversion circuit.